Sorbic acid (2,4-hexadienoic acid) and salts thereof, especially potassium sorbate, are widely used as antimicrobials in packaged foods and pharmaceutical and other health care products intended for human or animal use. Sorbic acid and salts thereof are particularly valuable as fungistatic agents to inhibit or retard the growth of molds and yeasts. Although sorbic acid and its salts are quite stable when dry, they are susceptible to oxidation in aqueous solution. This oxidation produces products such as ketones and aldehydes, which can cause development of undesired flavors or odors, and polymers of the aldehydes can cause development of undesired color. The oxidation of sorbic acid or salt thereof can result in the loss of sufficient sorbate ion to impair the desired antimicrobial effect.
The mechanism of oxidation of sorbic acid and means to stabilize against such oxidation have been the subject of considerable research. See, for instance, S. S. Arya et al., "Degradation Products of Sorbic Acid in Aqueous Solutions", Food Chemistry 29, (1988), pages 41-49; S. S. Arya, "Stability of Sorbic Acid in Aqueous Solutions", Journal of Agricultural and Food Chemistry, 28, (1980), 1246-1249; L. Pekkarinen et al., "THE EFFECTS OF SALTS OF HEAVY METALS ON THE STABILITY OF SORBIC ACID IN OXYGENATED SULPHURIC ACID SOLUTIONS", Suomen Kemistilehti, 40, No. 2, (1967), 54-58; L. Pekkarinen, "THE MECHANISM OF OXIDATION OF SORBIC ACID BY MOLECULAR OXYGEN IN WATER", Suomen Kemistilehti, 42, No. 3, (1969), 147-152; J. N. Sofos, "Sotbate Food Preservatives", CRC Press, Inc., Boca Raton, Fla. (1989); Pekkarinen, "The Influence of Metal Acetates on the Oxidation of Sorbic Acid by Molecular Oxygen in Acetic Acid and Comparison of the Results with Those for Eleostearic Acids", Acta Chemica Scandinavica, 26, (1972) 2367-2371; and Pekkarinen et al., "THE EFFECTS OF SALTS OF HEAVY METALS ON THE STABILITY OF SORBIC ACID IN OXYGENATED DILUTE SULPHURIC ACID SOLUTIONS", Suomen Kemistilehti B, 40, No 2., (1967) 54-58.
The above-cited authors have disclosed that heavy metal ions including manganous (Mn.sup.++) ion have been used to stabilize sorbic acid and sorbate salts against oxidation. The proportions disclosed to be effective have been 50 ppm (parts, by weight, per million) [S. S. Arya] and 10.sup.-2 to 10.sup.-3 mol/l [about 55 to 550 milligrams of manganous ion per liter - Pekkarinen et al.]
The present invention is based upon the discovery that the presence of a very small proportion of manganous ion in aqueous solutions of sorbic acid or salt thereof is effective in stabilizing such aqueous solutions against oxidation.